Various approaches are available for controlling air-fuel ratio of an internal combustion engine to reduce emissions. In one approach, a catalyst gain model and adaptive gain calibration are used to modify the control system action as the catalyst ages to reduce breakthrough of regulated emissions. Such an approach is described in U.S. Pat. No. 7,000,379.
However, the inventors herein have recognized an issue with such an approach. Specifically, under some conditions, the control system may be so accurate that even when catalyst operation degrades, the signals generated by available exhaust gas oxygen sensors may be indistinguishable from other noise.
As such in one approach, to perform catalyst diagnostics and/or to determine how the oxygen storage of the catalyst is changing as the system ages, a diagnostic monitor may override the air-fuel ratio control system. Specifically, the diagnostic monitor may intrusively schedule control system operation to increase the difference in oxygen (and/or reductant breakthrough (and thus, downstream air-fuel ratio sensor activity) so that a degraded catalyst can be more easily identified as compared to a sufficiently functioning catalyst. In one particular example, air-fuel ratio adjustment responsiveness to a sensor downstream of the monitored catalyst volume is temporarily increased from reduced settings used to improve air-fuel ratio control as the system ages.
In another approach, a system and method are described for controlling an internal combustion engine that include an inner feedback control loop to control the engine fuel/air (or air-fuel) ratio with feedback provided by a first exhaust gas sensor and an outer feedback control loop that modifies the fuel/air ratio reference provided to the inner feedback control loop based on feedback signals provided by the first exhaust gas sensor and a second exhaust gas sensor. The fuel/air ratio reference signal controller adapts to the oxygen storage capacity of the catalyst by modeling the catalyst as a nonlinear integrator with an unknown adaptive gain and estimating the catalyst gain based on the first and second exhaust gas sensor signals. An adaptive controller gain factor is then determined based on the estimated catalyst gain and is used to determine the fuel/air ratio reference signal. During catalyst diagnostic conditions, the effects of the adaptive controller are reduced or eliminated to enable improved signal to noise ratio in determining degradation due to catalyst degradation.
Such an approach enables adaptation of the fuel/air ratio controller to the changing oxygen storage capacity of the catalyst. The engine controller modulates the fuel/air ratio between limits set by a relay block within the outer loop controller as adjusted by an adaptive gain factor based on the estimated catalyst gain. The relay block switches between predetermined offset values of the fuel/air ratio reference depending on the sign of the error between the second exhaust gas sensor and its associated reference value, with the resulting fuel/air ratio reference provided to the inner loop controller. However, during catalyst diagnostic conditions, the adaptive gain factor can revert to a predetermined and fixed value (or fixed range of values) to thereby enable repeatable testing of the catalyst performance. In other words, as the catalyst ages, the adaptive control may effectively decrease the air-fuel ratio feedback gain relative to the downstream sensor (otherwise the system may be over-responsive to errors) to better match the aging catalyst. Then, during diagnostics, the gain may be temporarily increased to provide a higher signal to noise reading on the downstream sensor which can be used to provide a more accurate indicate of catalyst degradation.
Various advantages may be achieved by such an approach. For example, the present invention may provide a more robust engine controller that adapts to changing catalyst dynamics on-line, and thereby allows increased robustness to catalyst degradation. Further, to account for the negative impacts of such robustness on catalyst diagnosis, and intrusive approach may be used where control gains are temporarily increased to provide repeatable and accurate catalyst diagnostics, where the diagnostics are based on the downstream air-fuel ratio sensor.